Die casting machines are used to mold metallic articles. To do so, liquid metal is fed into the inner cavity of a mold where the metal hardens as it is cooled before the article is ejected from the mold. The process is repeated in a cycle to create numerous articles.
Molds typically comprise two mold portions that join at a so-called parting line that is in fact a plane along which mold surfaces of each mold portion engage one another. The mold portions both have recesses on these flat mold surfaces that form the inner mold cavity for liquid metal injection when the mold portions join. The mold portions can be separated to eject the metallic article once it is hard. In some molds, one of the mold portions is fixed and the other is mobile while in other instances, both mold portions are mobile. In both cases, the mold portions are movable relative to each other between a closed position in which the mold portions engage one another and an opened position in which the mold portions are spaced apart.
Die casting machines come in different types that are categorized in two groups: cold chamber die casting machines and hot chamber die casting machines.
A cold chamber die casting machine is typically used to mold aluminum pieces or sometimes pieces of another metal. In a cold chamber die casting machine, the injection sleeve (called the “shot sleeve”) is not partly submerged or otherwise surrounded by liquid metal. Rather, liquid metal is conveyed from a furnace located distally from the injection sleeve, to the injection sleeve, for example with a ladle that is operated by an automated arm or manually. The metal is consequently cyclically fed into the injection sleeve by this ladle before the ladle returns to the furnace to be refilled. While the ladle is being refilled, the injection sleeve injects the liquid metal into the mold cavity. A biscuit will desirably form at the inlet opening of the mold where the piston applies and maintains pressure against the metal while it hardens. The biscuit, usually of generally cylindrical shape, comprises hardened metal that is located partly in the shot sleeve and partly in the mold cavity at the cavity inlet opening, but that will not form part of the article being molded. Once the article is suitably hard, it will be ejected together with the biscuit, with the latter being disposed of for example by being returned to the furnace for the metal to be melted and reused.
A hot chamber die casting machine is typically used to mold zinc and magnesium pieces. It comprises a bath filled with molten liquid metal in which an injection sleeve, provided with a gooseneck in hot chamber die casting machines, is partly submerged. Liquid metal is allowed to cyclically flow into an inner chamber of the injection sleeve through an inlet opening to fill the inner chamber before a piston ejects the liquid metal out of the inner chamber and into the mold cavity, through an injection nozzle of the gooseneck provided on the injection sleeve. The liquid metal never completely hardens within the injection chamber or the injection nozzle and there is no formation of a biscuit in hot chamber die casting machines.
Hot chamber and cold chamber die casting machines each have respective operation characteristics, as known to those skilled in the art. It will be noted that principles that are applicable for hot chamber die casting machines are often not applicable for cold chamber die casting machines, and vice versa, due to differences in these operation characteristic. For example, the types of injection sleeves that are used differ (the injection sleeve in hot chamber die casting machines comprises a gooseneck having a nozzle while it doesn't in cold chamber die casting machines), the injection pressures differ, the formation of a biscuit in cold chamber die casting incurs cold-chamber specific requirements regarding injection pressures and mold-closing pressures; together with many other design and operation characteristics that are specific to the type of die casting machine—hot or cold—being used.
Concerning the biscuit mentioned above, it is noted that in cold chamber die-casting machines the metal will also harden within the passage called the runner which links the mold inlet opening to the gate, the latter being the entry point for the liquid metal into the mold cavity. So in fact, it is not only the biscuit that will be disposed of after the metal hardens, but also the diametrically smaller extraneous metal that extends between the biscuit and the gate in the runner. The gate is the point where the metal is distributed from the runner into the actual article cavity.
In known cold chamber dies casting machines, one platen is fixed while the other is mobile. The injection sleeve that injects the liquid metal into the mold cavity extends through the fixed platen and the corresponding fixed mold portion. One problem with prior art cold chamber die casting machines is linked to the feeding of liquid metal into the injection sleeve. Cold chamber injection sleeves comprise a piston movable within an elongated inner chamber, with the piston being capable of ejecting the liquid metal out through a liquid metal outlet port of the injection sleeve. In many cases, the injection sleeve is disposed horizontally and a liquid metal inlet port is provided atop the cylindrical sleeve, away from the outlet opening. The inner chamber is partly filled with liquid metal through the inlet opening, while the outlet opening is in fluid communication with the mold cavity. A problem with this configuration is that the horizontal disposition of the injection sleeve allows air to remain present in very significant proportion in the injection sleeve when the injection sleeve filling operation is completed but before the injection step starts. Indeed, the starting position of the piston will be the same notwithstanding the volume of injected liquid and consequently the injection sleeve will usually be filled only partly as a result of varying volumes being used to mold articles of different sizes. If the injection sleeve is half filled with molten metal, then it is also half filled with air. Consequently, a significant volume of air is often injected into the mold cavity concurrently with liquid metal, resulting in air bubbles being entrapped in the liquid metal in the mold. Although some known techniques exist to exhaust the entrapped air within the mold cavity, some air will often remain trapped, resulting in the metallic article comprising weakness zones once the metal is hardened where air bubbles are present.
Another injection sleeve configuration exists where the injection sleeve is provided with a single liquid metal port that is located at the free extremity of the injection sleeve and that is used both for filling the injection sleeve and for injecting the liquid metal into the mold. In this configuration, the injection sleeve is inclined to retain the liquid metal therein as it is being poured, thereby maximizing the volume of the injection sleeve inner chamber that is filled with liquid metal and consequently minimizing the volume of the injection sleeve that is occupied by air. However, this injection sleeve design suffers from at least one problem: the injection sleeve liquid metal port is located within the mold itself, forcing the feeding of the metal to be accomplished in an area between the two mold halves that is not easily accessible for a robotised arm. Significant design sacrifices have to be done on the die casting machine to accommodate such a configuration.
Positioning the injecting sleeve outside of the mold entirely and injecting the liquid metal at the parting line instead of between the two mold halves, has up to now not been seen as an operable or viable option in cold chamber die casting machines. Some prior art hot chamber die casting machines include injection at the parting line with the injection nozzle located outwardly of the mold. These hot chamber die casting machines have mold portions that close over the free metal liquid outlet extremity of the injection sleeve and the injection sleeve engages the mold at the parting line to inject the liquid metal parallel to and through the parting line into the mold cavity. In the case of hot chamber die casting machines, this design is possible since the injection force is relatively low.
However, in cold chamber die casting machines where injection forces are more important, this design with injection at the parting line is considered non-functional or non-practical and is not used in prior art devices to the knowledge of the present inventors. For one thing, the seal between the injection sleeve and the mold needs to be fluid-tight, as otherwise the liquid metal will be allowed to undesirably seep between the injection sleeve and the mold. To obtain a fluid-tight seal at important injection forces requires at the very least that the injection sleeve engage the mold at an important sleeve-mold sealing pressure. Since this sleeve-mold sealing pressure would be applied transversely of the mold, parallel to the parting line, this is likely to result in the mold being deformed by curving transversely unless the mold closing pressure is so important that it would counteract this deformation. Such a mold deformation would be undesirable since it would contribute to allowing the liquid metal to flash within the mold during the injection as a result of unevenly distributed pressure on the mold portions; while increasing the mold closing pressure significantly to counteract this deformation means that the die casting machine needs to be equipped with more expensive components in addition to more energy being expended to operate the die casting machine. In any event, by increasing the sleeve-mold sealing pressure, the mold is likely to wear down under the repeated pressured engagement of the sleeve on the mold. The mold wearing down at the junction area with the injection sleeve means that it might become uneven, resulting in the sleeve-mold sealing pressure being applied unevenly, in turn resulting in the metal seeping out between the injection sleeve and the mold during injection.
For these reasons and others, it has been considered common wisdom up to now to entirely avoid having a cold chamber die casting machine where the injection sleeve injects liquid metal at the parting line.
Air bubbles in the liquid metal also appear during the liquid metal pouring operation from the ladle into the injection sleeve, as a result of turbulence from the liquid metal inflow.
Another problem related to prior art die casting machines relates to liquid metal seeping outside of the inner mold cavity, between the mold portions. This undesirable seeping is called “flashing”. Reasons why the liquid metal flashes is because the pressure applied to keep the two mold portions pressed against each other is not important enough or is not well distributed along the parting line. One reason why this mold-closing pressure needs to be very important is to allow liquid metal injection at high pressure without the liquid metal flashing, the high pressure injection providing articles of higher quality.